1. Field of the Invention
The present invention relates to an optical scanning device and an image forming apparatus, and more particularly to an optical scanning device that scans a surface to be scanned with a light beam and to an image forming apparatus including the optical scanning device.
2. Description of the Related Art
In a digital copier, a laser printer, a laser facsimile, and the like, an image is written using an optical scanning device. Such an optical scanning device includes a light source having a light-emitting element, a first optical system that forms an image of a light beam output from the light source as a long linear image extending in a main-scanning direction, a deflector having a deflection reflecting surface disposed near a position where the linear image is formed to deflect a light beam output from the first optical system, and a second optical system that condenses a light beam deflected by the deflector to a spot of light on a surface to be scanned, and scans the surface with light beams. Also well known is a so-called multi-beam optical scanning device in which a surface to be scanned is scanned with a plurality of light beams by using a multi-beam light source having a plurality of light-emitting elements.
When a plurality (n) of light beams is used for scanning, the time required for forming an image is reduced to 1/n compared with a configuration using only one light beam, and thus, the multi-beam device contributes to an increase in speed of image formation. However, when the light intensities of the light beams are different, the concentration becomes uneven in each scanning line, resulting in lower image quality.
Therefore, an optical scanning device typically includes a detector, such as a photodiode, that detects part of the light beams output from the light source as monitoring light beams; and auto power control (APC) that controls the output level of the light source is performed based on the detection result.
An edge emitting laser diode (LD) that is a common semiconductor laser has a resonator that uses both edges of a crystal, formed on a substrate as reflecting surfaces. Front edge light is output from the front edge of the resonator, while rear edge light is output from the rear edge of the resonator.
A photodiode (PD) for receiving the rear edge light is typically incorporated in a given package with the edge emitting LD, and the amount of light output is controlled by feeding back the amount of light received by the PD.
On the contrary, a vertical cavity surface emitting laser (VCSEL) outputs light only in one direction. Therefore, unlike the edge emitting LD, the amount of light output cannot be monitored using the rear edge light. To address this issue, according to the inventions disclosed in Japanese Patent Application Laid-open No. H10-100476, Japanese Patent Application Laid-open No. 2002-26445, Japanese Patent Application Laid-open No. 2005-274678, and Japanese Patent Application Laid-open No. 2007-298563, for example, a light beam output from a VCSEL is split into a light beam for writing and a light beam for monitoring the amount of light.
However, the light amount control methods disclosed in Japanese Patent Application Laid-open No. H10-100476, Japanese Patent Application Laid-open No. 2002-26445, Japanese Patent Application Laid-open No. 2005-274678, and Japanese Patent Application Laid-open No. 2007-298563 may fail to obtain a sufficient amount of light for scanning a surface to be scanned. Therefore, Japanese Patent Application Laid-open No. 2009-145398 suggests using light beams that scan outside of a valid scanning area on the surface for controlling the amount of light.
An increasing number of molded plastic products has come to be used as optical elements in an optical scanning device, especially as a lens used in the second optical system (as a scanning lens) because the molded plastic products are economical and a free form curvature surface can be achieved relatively easily. Molded plastic scanning lenses are also widely adopted in multi-beam optical scanning devices in the same manner as in conventional optical scanning devices having a single-beam light source.
During a plastic molding process of an optical element, birefringence appears in lenses depending on their materials, production conditions, forms, and other factors. Birefringence is a phenomenon where the refractive index of a lens differs for each orthogonal light ray, and is expressed by a main axis orientation and a phase difference. The main axis orientation herein means the same as a fast axis orientation or a slow axis orientation.
Many scanning lenses are larger in size than pickup lenses (objective lenses) used in an optical disk apparatus, and some molded plastic scanning lenses have an uneven birefringence distribution inside the lenses. In particular, when the difference in thickness between the center and the peripheral of a lens is large, that is, when the thickness difference is larger, birefringence distribution will become more uneven.
For example, it is assumed herein that, as illustrated in FIG. 31, two light beams (a beam 1 and a beam 2), which are output from different light-emitting elements (ch1 and ch2) and kept separated from each other in the sub-scanning direction, pass through a scanning lens having a birefringence distribution illustrated in FIGS. 30A to 30C. In such an example, the birefringence of the scanning lens affects the beam 1 and the beam 2 differently. Therefore, as in an example illustrated in FIG. 32, the beam 1 and the beam 2, both of which are polarized linearly before being incident into the scanning lens, become polarized in different ways after passing through the scanning lens. In FIG. 32, the beam 1 is elliptically polarized in a vertically elongated manner, and the beam 2 is elliptically polarized in a horizontally elongated manner. If a folding mirror is disposed between the scanning lens and the surface to be scanned, for example, because the reflectance of the beam 1 and that of the beam 2 differ on the folding mirror, the amounts of light on the surface to be scanned also differ between ch1 and ch2. If the amounts of light on the surface to be scanned differ between the light-emitting elements, the concentration in an output image might become uneven, and especially, banding might occur.
The reason why the polarizations of ch1 and ch2 differ before and after passing through the scanning lens is that the scanning lens has an uneven birefringence distribution in the sub-scanning direction. Further, because the scanning lens also has an uneven birefringence distribution in the main-scanning direction, the uneven birefringence distribution in the sub-scanning direction differs depending on positions in the main-scanning direction. In other words, a difference in the amounts of light between ch1 and ch2 on the surface to be scanned differs from a difference in the amounts of light between ch1 and ch2 on the PD.
Therefore, the difference in the amounts of light between ch1 and ch2 on the surface to be scanned cannot be eliminated simply by controlling the amounts of light output from ch1 and ch2 based on the difference in the amounts of light on the PD. Such control could even increase the difference. In other words, in the conventional light amount controlling system, light beams are affected by birefringence twice.
The fact that light beams are affected by birefringence twice will now be explained. As illustrated in FIG. 33A, for example, it is assumed herein that the amounts of light output from ch1 and ch2 and being incident on a folding mirror are in a ratio of 100:100. Because the polarizations differ for ch1 and ch2 as illustrated in FIG. 32, the reflectance of the light beam 1 and that of the light beam 2 on the folding mirror also differ. Therefore, the amounts of light on the surface to be scanned would differ between ch1 and ch2, for example, in a ratio of 10 (ten percent reflectance):8 (eight percent reflectance). This is a conventional issue that is shared between an optical scanning device using an edge emitting LD and an optical scanning device using a VCSEL and including a monitoring PD disposed near a light source. However, if the amounts of light are monitored using the method disclosed in Japanese Patent Application Laid-open No. 2009-145398, this issue could become more severe.
Under such circumstances, because the birefringence distribution of the scanning lens differs depending on positions in the main-scanning direction, the polarizations of the scanning light beams travelling toward the PD also differ from the polarizations of writing light beams travelling toward the photosensitive element, and the amounts of light on the PD could be, for example, in a ratio of 7 (seven percent reflectance):10 (ten percent reflectance) between ch1 and ch2. At this time, if the amount of light output from ch2 is reduced in response to the amounts of light received by the PD and fed back to the light-emitting elements in an attempt to make the amounts of light on the PD even, the amounts of light on the PD would be in a ratio of 7:7 between ch1 and ch2 as illustrated in FIG. 33B, which means the amounts of light received by the both become even. However, the amounts of light on the photosensitive element would be in a ratio of 10:5.6 between ch1 and ch2, which means the difference between the amounts of light is rather increased.
The amounts of light can basically be controlled more accurately by monitoring the amounts of light near the surface to be scanned than by monitoring the amounts of light near the VCSEL. However, if a scanning lens having an uneven birefringence distribution is used in the scanning optical system, the accuracy in the monitored amounts of light is reduced.